Proprioceptive evoked gamma oscillations

A proprioceptive stimulus consisting of a weight change of a handheld load has recently been shown to elicit an evoked potential. Previously, somatosensory gamma oscillations have only been evoked by electrical stimuli. We conjectured that a natural proprioceptive stimulus also would be able to evoke gamma oscillations. EEG was recorded using 64 channels in 14 healthy subjects. In each of three runs a stimulus of 100 g load increment in each hand was presented in 120 trials. Data were wavelet transformed and runs collapsed. Inter-trial phase coherence (ITPC) was computed as the best measure of precision of evoked oscillations. The window of interest was 25-75 Hz and 0-200 ms. ITPC maxima were determined by visual inspection and these results were compared to results of multi-way factor analyses. The predicted 40 Hz activity was observed 60 ms after stimulus onset in the parietal region contralateral to stimulus side and additionally an unexpected 20 Hz activity was observed slightly lateralized in the frontal central region. The gamma phase locking may be a manifestation of early somatosensory feature integration. The analyses suggest that the high frequency activity consists of two distinct underlying oscillatory processes, in need of further investigation. (c) 2007 Elsevier B.V. All rights reserved.